This line echoes the Witches' incantation of the end of the first scene of the play:

Fair is foul and foul is fair:

Hover through the fog and filthy air.

So, for Macbeth to echo these words makes him appear to somehow be aligned with the Witches, which casts an evil and foreboding atmosphere around him.

The evil association would simply have come from the social assumption of the general population of Shakespeare's day that witches existed to perform evil deeds (the work of the Devil). Shakespeare didn't need to prove the evil of the witches to his audience, they would simply have assumed it. So, Macbeth's very first impression on the audience links him to the characters that represent evil in the play.

It is also an eerie foreboding that is created when Macbeth utters the words that the Witches have incanted. The suggestion that they have the power and ability to affect the future is implanted in the audiences mind at this moment, adding to the ominous foreboding of the atmosphere of Act One.

Please follow the links below for more on the Witches and how Elizabethans saw them and evil in the play.